Pipe-in-pipe pipelines, also known as PIP pipelines, are well known in the art, and generally comprise, with reference to the accompanying prior art FIG. 1, an outer pipe 10 (or “carrier pipe”) and a usually concentric or co-axial inner pipe or flowline 12. An annular space or “annulus” is defined between the inner and outer pipes. The inner pipe 12 is used to transport or convey fluids such as hydrocarbons, in particular oil and gas, between two or more static and/or moveable locations. This includes conveying fluids between vessels, or locations at or near a seabed such as an oil head, in particular a remote oil head, to an underwater facility and/or to a riser towards sea level, and/or directly to an onshore facility.
However, fluids circulating along deep-water or long tie-back flowlines may experience a significant temperature drop, leading to the risk of hydrate plugging and wax formation, especially in oil and gas flows. PIP pipelines can address the problem of hydrate or wax plugging without having to access the interior of the inner flowline. Active heating is typically implemented by a method known as ‘trace heating’, whereby electrical cables 14, which may be round or flat cables, are placed in the annular space between the inner and outer pipes to provide heat for maintaining the required temperature level of the fluid inside the flowline or inner pipe. A PIP pipeline with a trace heating arrangement is also known as an ETH-PIP pipeline (electrically trace heated pipe-in-pipe), and can involve flat shaped trace heating cables spiralled against the inner pipe, and under high performance thermal insulation. Electrical power can be fed from either a power umbilical or an Integrated Production Bundle (IPB) to a connection T-piece, located at one end of the ETH-PIP. Optical fibres can be included for real time temperature profile monitoring of the flowline. ETH-PIP pipelines are particularly well suited to long tie-back or heavy oil field applications because of low power consumption, which reduces both OPEX and topside CAPEX costs.
The manufacturing process of ETH-PIP pipelines is very similar to that of PIP pipelines. FIG. 1 shows an onshore manufacturing process in factory-like conditions. Generally, a number of PIP stalks, typically 1 to 1.5 km long, are assembled by insertion of the inner flowline inside the outer pipes. FIG. 1 shows a typical spoolbase layout for manufacturing an ETH-PIP, with the trace heating cables 14 being added to the flowline 12, which is then aligned with the insertion process into the outer or carrier pipe 10 to forma PIP stalk 16. Each PIP stalk 16 can then be joined with earlier PIP stalks already spooled onto a vessel, such as a reel lay vessel 18.
The process of joining two PIP stalks 16 together, sometimes termed a ‘tie-in’, begins with welding two inner flowlines 12 together. Then, the trace heating cables 14 and any fibre optic lines around the flowlines 12 of each PIP stalk 16 need to be connected. Currently, this connection process is carried out by directly splicing the cables 14 together. This is a time consuming process, generally taking at least 4 hours per cable. Where there are 3, 4 or more cables to be connected, it is appreciated that this can be a significant time requirement in the pipeline manufacture, which must occur for each tie-in process being carried out.